The DNA damages induced by carcinogens and their repair mechanisms are generally believed to be involved in cell transformation processes, but their direct roles are poorly understood at the molecular level. Variants subcloned from BALB/3T3 A31 cone 1 have been found to exhibit different sensitivity to chemical-induced transformation. The purpose of this work is to determine whether the excision repair of benzo(a)pyrene (BP)-damaged DNA (i.e. BP-deoxyguanosine) of these cells is responsible for their different response to chemicals. With the three classes of variants studied, it has been found that all variants excised carcinogen-damaged DNA with extremely low efficiency. They are very much like the cells obtained from Xeroderma Pigmentosum (XP) patients with respect to repair deficiency. Two immunological methods have been explored to assay the minute amount of BP-DNA formed in human KD and XP cells.